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Grouting of the post-tensioned cables in the post or prestressed concrete is carried out to provide permanent protection to the steel cables against corrosion. It also develops a bond between cables and surrounding concrete which fills the duct space and prevents water collection and freezing.
In this article we discuss, materials, equipment, properties, and procedure of grouting in post-tensioned cables.
1. Water, Cement, and Sand
The water used in the preparation of the grout will be clean potable water with no dissolved impurities. Sea water is not used in the preparation of grout.
OPC 43 cement is preferable used in the preparation of grout which must be as fresh as possible and free from lumps. Pozzolana cement shall not be used.
The use of sand is not recommended for the grouting of prestressing tendons. In case the internal diameter of the ducts exceeds 150 mm, the sand used must conform to IS:383 and shall pass through IS Sieve No. 150.
The weight of sand in the grout shall not be more than 10% of the weight of cement unless proper workability can be ensured by the addition of suitable plasticizers.
An admixture that improves the properties of grout, i.e. increasing fluidity, reducing bleeding entraining air or expanding the grout is recommended for use in the preparation of grout for post-tensioning cable.
The admixture used in grout must conform to IS: 9102 which must be free from chlorides, nitrates, sulfides, sulfites or any other products which are likely to damage the steel or grout.
1. Grout Mixer and Agitator
Obtaining a homogeneous, uniform consistency and colloidal grout so that there is no separation of cement is essential. The mixer should have an additional storage device with an agitator to keep the grout moving continuously before it is pumped in the duct.
2. Grout Pump
The pump used to supply the grout into the clabes must be positive displacement type and should be capable of ejecting the grout in a continuous operation and not by way of pulses.
A pressure guage is fixed at the outer part of the pump to measure the pressure at which the injection is controlled. The minimum pressure at which grout should be pumped shall be 0.3 MPa.
Properties of Grout
- The water/cement ratio should be as low as possible, consistent workability. This ratio should not normally exceed 0.45.
- The properties of the grout mix should be tested in a laboratory depending on the facilities available before the grouting process is undertaken.
- Tests should be conducted for each job periodically. The recommended test is described below.
- Compressive Strength: The compressive strength of 100 mm cubes of the grout shall not be less than 17 MPa at 7 days.
The grouting procedure shall be carried not later than 2 weeks of stressing. The anchorage ends of the cables are sealed with same grade of concrete after the stressing is completed.
1. Pre-Grouting Procedures
- If any traces of oils are found applied on the steel for preventing corrosion, they should be removed before grouting operation.
- For cleaning and wetting of the surfaces of the duct walls, clean potable water is flushed into the ducts.
- The water should be drained through the lowest vent pipe or by blowing oil-free compressed air through the duct.
- All outlet points including vent openings should be kept open prior to commencement of injection grout.
- Before grouting process, all air in the pump and hose should be expelled. The suction circuit of the pump should be airtight.
2. Injection of Grout
- The mixed grout is continuously agitated and should be left stagnant.
- The process of injection of grout must be continuous and should not be interrupted.
- Grouting should be commenced initially with a low pressure of injection of up to 0.3 MPa increasing it until the grout comes out at the other end.
- The grout should be allowed to flow freely from the other end until the consistency of the grout at this end is the same as that of the grout at the injection end.
- When the grout flows at the other end, it should be closed off and building up of pressure commenced. Full injection pressure at about 0.5 MPa is maintained for at least 1 min to ensure effective grouting.
- Grout not used within 30 minutes of mixing should be rejected.
- For vertical cable or cables inclined more than 66 degrees to the horizontal, the injection should be effected from the lowest anchorage or vent of the duct.
- The method of injection should ensure complete filling of the ducts. To verify this, it is advisable to compare the volume of the space to be fitted by the injected grout with the quantity of grout actually injected.
1. 10 Precautions for Effective Grouting in Tensioning Ducts
2. Pre-Tensioning and Post-Tensioning in Prestressed Concrete Design
3. Post Tension Slab – Working Principle, Components and Construction